1. Field of the Invention
The present invention relates to methods for thermally treating dielectric films in integrated circuits. More particularly, the present invention relates to methods of annealing dielectric films in the presence of water vapor to improve their thermal stability and to improve their resistance to damage from ultraviolet radiation and their recovery from damage caused by ultraviolet radiation.
2. Description of the Related Art
Capacitive coupling between metal lines in an integrated circuit increases in proportion to the dielectric constant of the intermetal layers, and increases in inverse proportion with the distance between metal lines. As the typical feature size in integrated circuits continues to decrease with each new generation of circuits, the spacing between metal lines in the circuits also decreases. Consequently, as integrated circuits increase in complexity and shrink in size, capacitive coupling between metal lines increases in magnitude. The signal delays associated with capacitive coupling similarly grow in magnitude, and degrade the performance of the circuits.
The capacitive coupling between metal lines may be decreased by reducing the dielectric constant of the intermetal layer. The intermetal layers are typically composed of SiO.sub.2, which has a dielectric constant of approximately 4. Thus, industry is searching for materials with dielectric constant less than 4 which may be used in the intermetal layers of integrated circuits.
Several low dielectric constant materials have been investigated as substitutes for SiO.sub.2 in integrated circuit intermetal layers. Examples of materials investigated as substitutes for SiO.sub.2 include polyimides, polytetrafluoroethylene, parylenes, polysilsesquioxanes, fluorinated poly(aryl ethers), fluorinated amorphous carbon, organo silicate glasses available under the tradename CORAL.TM. from Novellus Systems Inc., and organo silicate glasses available under the tradename Black Diamond.TM. from Applied Materials Inc.
Intermetal layers composed of SiO.sub.2 generally have favorable physical characteristics. For example, SiO.sub.2 layers typically have good mechanical stability at elevated temperatures and good resistance to damage from ultraviolet radiation. These characteristics are advantageous, as the integrated circuit manufacturing process typically involves repeated exposure to ultraviolet radiation and to temperatures of at least 400.degree. C. Exposure of the dielectric material to ultraviolet radiation may be intentional, or may be incidental to deposition, etching, or other processing of the wafer.
The performance of low dielectric constant material substitutes for SiO.sub.2 in intermetal layers has not been entirely satisfactory. Particularly for organic dielectric films, deficiencies of these materials may include poor stability at elevated temperature, and high susceptibility to damage by exposure to ultraviolet radiation.
Dielectric intermetal layers may be annealed in an attempt to improve their performance. Since the presence of water in an intermetal layer may degrade the performance of an integrated circuit, in the prior art, annealing of the dielectric intermetal layers is typically done in the absence of water or with water present in the ambient environment at less than one part per million. Annealing low dielectric constant materials in a water free atmosphere generally fails to produce films that are entirely satisfactory substitutes for SiO.sub.2 intermetal layers.
Accordingly, what is desired is a method for treating dielectric films to improve their thermal stability and to improve their resistance to damage from ultraviolet radiation.